Adjustable focal length illuminator for a display panel

ABSTRACT

An illuminator for a display panel includes a slab of transparent material for propagating illuminating light between outer surfaces of the slab, an out-coupler supported by the slab for out-coupling portions of the illuminating light along one of the outer surfaces of the slab, and a tunable microlens array for forming an array of light spots from the out-coupled illuminating light portions downstream of the focusing element for illuminating pixels of the display panel. The array of light spots may be repeated at a distance from the tunable microlens array due to Talbot effect. The display panel may be illuminated in a color-sequential manner, and the tunable microlens array may be used to adjust the focal plane position for each color channel individually.

REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication No. 63/286,381 entitled “Display Applications of SwitchableGratings”, and U.S. Provisional Patent Application No. 63/286,230entitled “Active Fluidic Optical Element”, both filed on Dec. 6, 2021and incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to illuminators, visual display devices,and related components, modules, and methods.

BACKGROUND

Visual displays provide information to viewer(s) including still images,video, data, etc. Visual displays have applications in diverse fieldsincluding entertainment, education, engineering, science, professionaltraining, advertising, to name just a few examples. Some visual displayssuch as TV sets display images to several users, and some visual displaysystems such s near-eye displays (NEDs) are intended for individualusers.

An artificial reality system generally includes an NED (e.g., a headsetor a pair of glasses) configured to present content to a user. Thenear-eye display may display virtual objects or combine images of realobjects with virtual objects, as in virtual reality (VR), augmentedreality (AR), or mixed reality (MR) applications. For example, in an ARsystem, a user may view images of virtual objects (e.g.,computer-generated images (CGIs)) superimposed with the surroundingenvironment by seeing through a “combiner” component. The combiner of awearable display is typically transparent to external light but includessome light routing optic to direct the display light into the user'sfield of view.

Because a display of HMD or NED is usually worn on the head of a user, alarge, bulky, unbalanced, and/or heavy display device with a heavybattery would be cumbersome and uncomfortable for the user to wear.Consequently, head-mounted display devices can benefit from a compactand efficient configuration, including efficient light sources andilluminators providing illumination of a display panel, high-throughputocular lenses and other optical elements in the image forming train.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A is a side cross-sectional view of an illuminator of thisdisclosure;

FIG. 1B is a side cross-sectional view of a Talbot embodiment of theilluminator of FIG. 1A, superimposed with lateral optical power densitydistribution of the illuminating light;

FIG. 2 is a Talbot distribution of optical power density in a substrateof the display panel illuminated by the illuminator of FIG. 1A, inaccordance with an embodiment;

FIG. 3 is a side cross-sectional view of a lightguide illuminated withlight of three color channels at different angle of incidence,illustrating a color-specific lateral shift of light spots focused bythe focusing element of the illuminator of FIG. 1A;

FIGS. 4A to 4D are side cross-sectional view of a tunable microlensarray of this disclosure in a color-sequential illuminationconfiguration;

FIG. 5 is a side cross-sectional view of a microlens array with atunable liquid crystal (LC) layer;

FIG. 6 is a side cross-sectional view of a tunable LC droplet microlensarray;

FIG. 7A is a plan view of an array of tunable Pancharatnam-Berry phase(PBP) LC microlenses;

FIG. 7B is a frontal view of a single PBP LC microlens of the array ofFIG. 7A;

FIG. 7C is a magnified schematic view of LC molecules in an LC layer ofthe tunable PBP LC microlens of FIG. 7B;

FIG. 8 is an exploded vide of a display apparatus with tunable thepositions of the array of light spots/optical power density peaks of theilluminating light;

FIG. 9 is a flow chart of a method for illuminating a display panel inaccordance with this disclosure;

FIG. 10 is a view of wearable display of this disclosure having a formfactor of a pair of eyeglasses; and

FIG. 11 is a three-dimensional view of a head-mounted display (HMD) ofthis disclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated.

In a visual display including an array of pixels coupled to anilluminator, the efficiency of light utilization depends on a ratio of ageometrical area occupied by pixels to a total area of the displaypanel. For miniature displays often used in near-eye and/or head-mounteddisplays, the ratio can be lower than 50%. The efficient backlightutilization can be further hindered by color filters on the displaypanel, which on average transmit no more than 30% of incoming light. Ontop of that, there may exist a 50% polarization loss forpolarization-based display panels such as liquid crystal (LC) displaypanels. All these factors considerably reduce the light utilization andoverall wall plug efficiency of the display, which is undesirable.

In accordance with this disclosure, light utilization and wall plugefficiency of a backlit display may be improved by providing alightguide illuminator coupled to a microlens array. The microlens arrayis disposed downstream of the slab lightguide to concentrate theout-coupled wide light beam into an array of tightly focused lightspots. In displays where the illuminator emits light of primary colors,e.g. red, green, and blue, the colors and locations of focused spots ofilluminating light may be matched to that of the color filters of thedisplay. Furthermore, upon illumination with color-interleaved arrays offocused spots, the color filters may be omitted altogether. Forpolarization-based displays, the polarization of the emitted light maybe matched to a pre-defined input polarization state. Matching thespatial distribution, transmission wavelength, and/or the transmittedpolarization characteristics of the pixels of the display panel enablesone to considerably improve the useful portion of display light that isnot absorbed or reflected by the display panel on its way to the eyes ofthe viewer, and consequently to considerably improve the display's wallplug efficiency.

The microlens array used to provide the array of illuminating spotsmatching the array of pixels may have a strong chromatic focal shift,especially for high numerical aperture of the microlenses used toprovide a wide viewing angle of the display panel. Furthermore, in someembodiments, a light interference effect termed Talbot effect may beutilized to have the illuminating light cross a comparatively thicksubstrate of the display panel while preserving the high numericalaperture of the focused light spots. A Talbot distance, i.e. a distancewhere a peaked illuminating pattern is repeated, depends on wavelength.The Talbot distance wavelength dependence may result in light spots ofdifferent colors forming at different depths into the display panelsubstrate.

In accordance with this disclosure, the color dependence of the focallength and/or the color dependence of Talbot distance may be overcome byproviding an array of tunable microlenses, i.e. microlenses having atunable or switchable focal length. The lightguide may be fed with lightof different color channels in a color-sequential manner, and the focallength of the microlens array may be tuned or switched in sync withsequencing the colors, enabling all colors of the illuminating light tobe focused effectively into a display panel. Throughout thespecification, the terms “tunable” and “switchable”, when applied tolenses or other focusing elements, are used interchangeably.

In accordance with the present disclosure, there is provided anilluminator comprising a slab of transparent material, the slabincluding opposed first and second surfaces for propagating illuminatinglight in the slab by a series of internal reflections from the first andsecond surfaces. An out-coupler is supported by the slab forout-coupling portions of the illuminating light from the slab at thefirst surface. A tunable microlens array is coupled to the first surfacefor forming an array of light spots from the out-coupled illuminatinglight portions at an adjustable distance from the first surface.

The illuminator may further include a multi-color light source forproviding the illuminating light of a color channel of a plurality ofcolor channels, and an in-coupler for in-coupling the illuminating lightinto the slab. The in-coupler may be configured to in-couple differentcolor channels of the plurality of color channels at different angles,whereby a lateral position of light spots of the array of light spotsdepends on the color channel of the illuminating light. The tunablemicrolens array may include at least one of: a liquid crystal layer witha variable liquid crystal orientation; a tunable liquid crystalmicrolens array; an array of switchable Pancharatnam-Berry phasemicrolenses; etc.

In accordance with the present disclosure, there is provided a displayapparatus comprising a display panel including a pixel array on asubstrate, and an illuminator described above. In operation, the anarray of light spots may be formed on the pixel array. In someembodiments, light of the array of light spots propagates through thesubstrate and produces an array of optical power density peaks at thepixel array due to Talbot effect. The pixel array may include aplurality of interleaved color sub-pixel arrays, each color sub-pixelarray corresponding to a color channel of a plurality of color channelsof an image to be displayed by the display apparatus.

The illuminator may include a multi-color light source for providing theilluminating light of a color channel of the plurality of colorchannels, and an in-coupler for in-coupling the illuminating light intothe slab. The in-coupler may be configured to in-couple different colorchannels of the plurality of color channels at different angles, wherebya lateral position of light spots of the array of light spots depends onthe color channel of the illuminating light. In operation, light of thearray of light spots may propagate through the substrate and produce anarray of optical power density peaks at the pixel array due to Talboteffect. A lateral position of optical power density peaks of the arrayof optical power density peaks may be matched to a lateral position of acorresponding color sub-pixel sub-array of the plurality of interleavedcolor sub-pixel arrays.

In some embodiments, the display apparatus may further include acontroller operably coupled to the multi-color light source and thetunable microlens array. The controller may be configured to operate themulti-color light source to provide the illuminating light in acolor-sequential manner, and to tune the tunable microlens array toadjust the distance depending on a current color channel of theilluminating light. The in-coupler may include a tiltable reflector forvarying an angle of incidence of the illuminating light onto the slab.

In accordance with the present disclosure, there is further provided amethod for illuminating a display panel comprising a pixel array on asubstrate. The method includes propagating illuminating light in a slabof transparent material by a series of internal reflections from opposedfirst and second surfaces of the slab; out-coupling portions of theilluminating light from the slab at the first surface using anout-coupler; focusing the out-coupled illuminating light portions at adistance from the first surface using a tunable microlens array; andtuning the tunable microlens array to form an array of light spots forilluminating the pixel array of the display panel.

The method may further include propagating light of the array of lightspots through the substrate to produce an array of optical power densitypeaks at the pixel array due to Talbot effect. The method may furtherinclude operating a multi-color light source to provide the illuminatinglight in a color-sequential manner, and tuning the tunable microlensarray to adjust the distance depending on a current color of theilluminating light. The method may further include in-coupling differentcolor channels of the plurality of color channels at different angles,whereby a lateral position of light spots of the array of light spotsdepends on a color channel of the illuminating light.

In embodiments where the pixel array comprises a plurality ofinterleaved color sub-pixel arrays, the method may further includepropagating light of the array of light spots through the substrate toproduce an array of optical power density peaks at the pixel array dueto Talbot effect, and matching a lateral position of the optical powerdensity peaks of the array of optical power density peaks to a lateralposition of a corresponding color sub-pixel sub-array of the pluralityof interleaved color sub-pixel arrays.

Referring now to FIG. 1A, an illuminator 100 is configured forilluminating a display panel 102. The display panel 102 includes asubstrate 122 supporting an array of pixels 124 (e.g. one-dimensional ortwo-dimensional array) and an optional black grid 130. The display panel102 may include other layers and substrates, which are omitted in FIGS.1A and 1B for brevity of illustration. The illuminator 100 includes aslab 104 of transparent material, e.g. a plano-parallel slab of glass,plastic, transparent oxide, transparent crystalline material, or anothersuitable material. The slab 104 includes first 111 and second 112opposed surfaces, which in this example are outer surfaces of the slab104 disposed parallel to XY plane, for propagating illuminating light106 in the slab 104 by a series of internal reflections, e.g. totalinternal reflections, from the first 111 and second 112 opposedsurfaces. The series of internal reflections is illustratedschematically with a zigzag dashed line, which represents thepropagation path of the illuminating light 106.

The illuminating light 106 is emitted by a light source 108. Theilluminating light 106 is coupled into the slab 104 by an in-couplinggrating 110 or by another suitable in-coupler such as a prism, a slantedsurface, etc. Portions 114 of the illuminating light 106 propagating inthe slab 104 are out-coupled through the first surface 111 by anout-coupler, e.g. an out-coupling grating 116, supported by the slab104. The out-coupling grating 116 may be a smooth and flat, continuousgrating, and may be disposed in the slab 104 or on the slab 104, asshown in FIG. 1A. The slab 104 with its out-coupling grating 116operates as a pupil-replicating lightguide providing multiple offsetlight portions 114 of the illuminating light 106. The out-coupled lightportions 114 form a nearly-collimated wide light beam that impinges ontoa tunable microlens array 118, which forms an array of light spots 120(FIG. 1B). In other words, the array of light spots 120 is formed byfocusing the out-coupled illuminating light portions 114 by the tunablemicrolens array 118. The light spots 120 may be formed at an adjustabledistance or depth, i.e. at an adjustable Z-coordinate in FIGS. 1A and1B, by tuning the focal distance of the tunable microlens array 118.

In embodiments where the substrate 122 of the display panel 102 is thinenough, the light spots 120 may illuminate the pixels 124 of the displaypanel 102 directly. In embodiments where the substrate 122 is too thickfor the light spots 120 to be smaller than the pixel 124 width and/or tobe formed with a sufficient angle of convergence at the pixel center,the tunable microlens array 118 may be configured to provide an array ofoptical power density peaks 128 from the array of light spots 120 at az-distance from the array of light spots 120 by utilizing Talbot effect.The Talbot effect causes a peaky optical power density distribution toreproduce itself at a distance from the array of light spots 120 forsufficiently spatially coherent light. In FIG. 1B, the array of opticalpower density peaks 128 illuminates the pixels 124. Positions ofindividual optical power density peaks 128 are coordinated withpositions of individual pixels 124 of the display panel 102, with oneoptical power density peak 128 illuminating one pixel 124 in thisexample.

The Talbot effect that reproduces the optical power density distributionat a higher plane spaced apart from the original plane of a peakyoptical power density distribution is illustrated in FIG. 2 . Thisfigure shows a map 200 of optical power density through the substrate122 of the display panel 102, with horizontal axis (i.e. X-axis in FIG.2 ) representing a lateral coordinate on the tunable microlens array118, and a vertical axis (i.e. Z-axis in FIG. 2 ) representing thethickness dimension of the substrate 122 of the display panel 102. Thetunable microlens array 118 may be configured to form the array of lightspots 120 at a focal plane 202 disposed some 0.09 mm into the substrate122. The lateral (XY) optical power density distribution is repeated ata Talbot plane 204, forming the array of optical power density peaks 128at the Talbot plane 204 with a same pitch as at the focal plane 202. Thearray of pixels 124 of the display panel 102 may be disposed at theTalbot plane 204. Such a configuration allows the efficiency of lightutilization to be considerably increased due to most of the illuminatinglight 106 propagating through the pixels 124 without being absorbed bythe black grid 130 between the pixels 124 (FIGS. 1A and 1B).

One method to provide an array of color-dispersed light spots forilluminating of a color display panel including a plurality ofinterleaved color sub-pixel arrays is to pre-tilt light beams ofindividual color channels impinging onto the pupil-replicatinglightguide. Referring to FIG. 3 for a non-limiting illustrative example,the slab 104 is illuminated by a multi-color light source 306 providinglight of red (R), green (G), and/or blue (B) color channels in-coupledby the in-coupling grating 110 into the slab 104 at slightly differentin-coupling angles. The in-coupling angles are exaggerated in FIG. 3 forclarity. A light beam of the R color channel is shown with long-dashlines, a light beam of the G color channel is shown with solid lines,and a light beam of B color channel is shown with short-dash lines. Toprovide different incidence angles for the in-coupled light of differentcolors, the multi-color light source may include a plurality oflaterally offset laser diodes and/or light-emitting diodes of differentcolors coupled to a common collimator. The multi-color light source 306may provide light of different colors simultaneously or in acolor-sequential manner.

The light beams of the R, G, B in-coupled color channels propagate inthe slab 104. The light beams are out-coupled by the out-couplinggrating 116 at angles corresponding to the in-coupling angles of the R,G, B light beams into the slab 104. Since the light beams of the R, G, Bin-coupled color channels are out-coupled at different angles, theout-coupled light beams of the R, G, B color channels are focused by thetunable microlens array 118 at offset locations formingcolor-interleaved sub-arrays of R, G, B light spots, as illustrated inFIG. 3 . Each one of the color-interleaved sub-arrays of R, G, B lightspots corresponding to light of a particular one of the plurality of R,G, B color channels. The configuration of FIG. 3 may be used forilluminating the interleaved color sub-pixel arrays of a color displaypanel directly, or by using the Talbot effect.

The utilization of Talbot effect for color panel illumination requiresaccounting for a dependence of z-position Z_(T) of a Talbot order N onwavelength, which may be defined by the following equation:

$\begin{matrix}{{Z_{T} = \frac{{Na}^{2}}{\lambda}},} & (1)\end{matrix}$

where N is an integer denoting a Talbot order, a is the length of aTalbot period and λ is wavelength of light. The z-positions Z_(T) ofdifferent color channels (i.e. different wavelengths) are different fora same Talbot order N. The focal length tunability of the microlensarray 118 enables the Z-coordinate of the focal plane of thecolor-interleaved sub-arrays of R, G, B light spots to be dynamicallyvaried. This may be useful to precisely focus the R, G, B light spotsonto the pixel array of the display panel being illuminated. Incolor-sequential illumination configurations with only illuminatingcolor present at any given moment of time, the Z-coordinate of the focalplane may be adjusted in a color-selective manner, enabling a greatdegree of flexibility in compensating the inherent dependence of Talbotdistance on wavelength represented by Eq. (1) above, as well asaccounting for a chromatic focal shift of microlens arrays due tochromatic dispersion of the microlens material.

The latter point is illustrated in to FIGS. 4A to 4D. In FIG. 4A, themicrolens array 118 is not tuned as the color composition of theilluminating light is changed. Due to chromatic dispersion of themicrolens material, light 406B of blue color is focused more stronglythan light 406G of green color and light 406R of red color. Themicrolens array 118 may be tuned to adjust the focal distance dependingon a current color channel of the illuminating light. This isillustrated in FIGS. 4B to 4D, where the microlens array 118 is tuned toadjust all focal lengths f to be equal, that is, the focal length forthe blue color light 406B equal to the focal length for the green colorlight 406G equal to the focal length for the red color light 406R. Thetunability of the microlens array 118 enables compensation of anychromatic focus shifts, however caused, be it due to materialdispersion, Talbot distance dependence on wavelength as given by Eq. (1)above, or for any other reason, provided a sufficient tuning range ofthe microlens array 118.

Several illustrative non-limiting examples of tunable microlens arraysusable in illuminators of this disclosure will now be considered.

Referring first to FIG. 5 , a microlens array 518 may be used as thetunable microlens array 118 of the illuminator 100 of FIGS. 1A and 1B.The microlens array 518 includes an array of refractive microlenses 509made of isotropic or anisotropic material, e.g. isotropic or anisotropicpolymer. The refractive microlenses 509 are disposed in a liquid crystal(LC) cell defined by first 501 and second 502 opposed transparentelectrodes. The LC cell is filled with a nematic LC fluid, forming an LClayer 505. The LC layer 505 includes rod-like LC molecules 504 that maybe oriented in an electric field caused by applying voltage to the first501 and second 502 electrodes. The nematic LC fluid is birefringent withan optic axis parallel to LC director. The LC director indicates a localorientation of the LC molecules 504. Typically, an effective refractiveindex is higher for light linearly polarized along the LC director. Inoperation, application of voltage to the first 501 and second 502electrodes causes the LC molecules 504A of the LC layer 505 to becomevertically oriented, i.e. oriented along Z-axis as shown in FIG. 5 ,causing a reduction of the effective refractive index of the LC layer505 surrounding the refractive microlenses 509, which causes theireffective focal length to be tuned.

Turning to FIG. 6 , a tunable LC microlens array 618 may be used as thetunable microlens array 118 of the illuminator 100 of FIGS. 1A and 1B.The tunable LC microlens array 618 of FIG. 6 may include an array of LCmicrolenses 600 including round droplets (e.g. hemispherical droplets)of oriented LC molecules 604 immersed into an isotropic polymersubstrate 603. The refractive index of the isotropic polymer substrate603 may be matched to an ordinary index of refraction of the LC fluid inthe droplets, or may be different from both the ordinary andextraordinary indices of refraction of the LC fluid in the droplets. TheLC molecules 604 may be oriented e.g. along X-axis as shown on aleft-side portion of FIG. 6 . When illuminated with a light beam 606linearly polarized along x-axis, the microlens 600 will focus the lightbeam 306 due to the focusing property of a curved interface 611 betweenthe LC droplets and the polymer substrate 603, the curved interface 611having a non-zero refractive index step, similarly to the microlensarray 518 of FIG. 5 . When the LC molecules 604 become oriented alongZ-axis, e.g. by applying a strong voltage to a pair of optionaltransparent electrodes 614 and 615, the curved interface 611 has a zerorefractive index step since the refractive index of the isotropicpolymer substrate is matched to an ordinary index of refraction of theLC fluid. Accordingly, the light beam 606 will remain non-focused asillustrated on the right-side portion of FIG. 6 . When the appliedvoltage is insufficient to orient the LC molecules 604 along Z-axis, orwhen the refractive index of the isotropic polymer substrate 603 isdifferent from both the ordinary and extraordinary indices of refractionof the LC fluid in the droplets, applying the electric field will changethe focal length of the LC microlenses 600, while retaining somefocusing/defocusing power of the LC microlens array 618.

Referring now to FIG. 7A, a Pancharatnam-Berry phase (PBP) microlensarray 718 may be used as the tunable microlens array 118 of theilluminator 100 of FIGS. 1A and 1B. The PBP microlens array 718 includesan array of PBP LC microlenses 700 formed in a liquid crystal (LC) layer702 including LC molecules 704 (FIGS. 7A, 7B). The LC molecules 704 aredisposed in XY plane at a varying in-plane orientation depending on thedistance r from the lens center. The orientation angle ϕ(r) of the LCmolecules 704 in the liquid crystal layer of each PBP LC microlens 700is given by

$\begin{matrix}{{\phi(r)} = \frac{\pi r^{2}}{2f_{0}\lambda_{0}}} & \left( {2a} \right)\end{matrix}$

where f₀ is a desired focal length and λ₀ is wavelength. The opticalphase delay in each PBP LC microlens 700 is due to Pancharatnam-Berryphase, or geometrical phase effect. An optical retardation R of theliquid crystal layer having a thickness t is defined as R=tΔn, where Δnis the optical birefringence of the liquid crystal layer. At the opticalretardation R of the LC layer of λ_(∥)/2, i.e. half wavelength, theaccumulated phase delay P(r) due to the PBP effect can be expressedrather simply as P(r)=2ϕ(r), or, by taking into account Eq. (2a) above,

$\begin{matrix}{{P(r)} = \frac{\pi r^{2}}{f_{0}\lambda_{0}}} & \left( {2b} \right)\end{matrix}$

It is the quadratic dependence of the PBP P(r) on the radial coordinater that results in the focusing, or defocusing, function of each PBP LCmicrolens 700. Each PBP LC microlens 300B has the azimuthal angle ϕcontinuously and smoothly varying across the surface of the LC layer(FIG. 7C). Accordingly, the mapping of the azimuthal angle to PBP, i.e.P(r)=2ϕ(r) when R=λ−/2, allows for a more drastic phase change withoutintroducing discontinuities at a boundary of 2π modulo.

The phase delay relationship as defined by Eq. (2b) may be erased byapplication of an electric field across the LC layer 702, making the PBPLC microlens 700 switchable. By combining the switchable PBP LCmicrolens 700 with a constant refractive microlens, and/or by providinga stack of the switchable PBP LC microlenses 700 of different opticalpowers, the tunability of the PBP LC microlens array 718 may beexpanded.

Referring to FIG. 8 , a display apparatus 850 is a non-limitingillustrative embodiment of application of an illuminator of thisdisclosure in a visual display. The display apparatus 850 includes thedisplay panel 102 having the array of pixels 124 defined by the blackgrid 130 and supported by the substrate 122, and an illuminator 800coupled to the display panel 102 for illuminating the array of pixels124 through the substrate 122.

The illuminator 800 is an embodiment of the illuminator 100 of FIG. 1A,and includes similar elements. The illuminator 800 of FIG. 8 includes aslab 804 of transparent material, an in-coupling grating 810 and anout-coupling grating 816 supported by the slab 804. The in-couplinggrating 810 is a polarization volume hologram (PVH) grating thatdiffracts circularly polarized light of a first handedness whiletransmitting through a circularly polarized light of a second, oppositehandedness.

In operation, a light source 808 emits a light beam 806 that iscircularly polarized at the second handedness. The light beam 806propagates through the PVH in-coupling grating 810 and the slab 804 andimpinges onto a microelectromechanical system (MEMS) 840 including atiltable reflector 842. The light beam 806 is reflected by the tiltablereflector 842 and impinges again onto the PVH in-coupling grating 810.Since handedness of a circularly polarized light reverses uponreflection, the reflected light beam 806 is diffracted by the PVHin-coupling grating 810, which in-couples the light beam 806 into theslab 804 to propagate in the slab 804 by a series of internalreflections from opposed outer surfaces of the slab 804, as illustratedwith a solid arrowed zigzag line.

Out-coupled portions 814 of the light beam 806 are focused by a tunablemicrolens array 818, which may include any of the tunable microlensesdescribed above with reference to FIGS. 4A-4D to FIGS. 7A-7C. It is tobe noted that the out-coupled portions 814 originate from an out-coupledwide beam that is broken into individual portions or sub-beams bymicrolenses 819 of the tunable microlens array 818, not necessarily bythe X-period of zigzag reflections. The individual portions are focusedinto light spots 820, similarly to the illuminator 100 of FIG. 1A. Thearray of light spots 820 is converted into an array of optical powerdensity peaks 828 by Talbot effect in an optical stack including thesubstrate 122 of the display panel 102, as explained above withreference to FIGS. 1A, 1B, and FIG. 2 .

The illuminator 800 of FIG. 8 further includes a controller 880 operablycoupled to the light source 808, the tunable microlens array 818, andthe MEMS 840. The controller 880 is configured to tilt the tiltablereflector 842 by a controllable angle to make the array of optical powerdensity peaks 828 coincide with the array of pixels 124 defined by theblack grid or black matrix 130. For example, initially the controller880 may tilt the out-coupled portions 814 to shifted positions 814′shown with dashed lines. At the shifted positions 814′, the light spots820 are shifted from a nominal position. The shifted light spots 820cause the array of optical power density peaks 828 to also be shifted topositions 828′ shown with dashed lines, and no light passes through theblack grid 130. The controller 880 may tune the angle of tilt of thetiltable mirror 842 to bring the array of optical power density peaks828 to the positions overlapping with the pixels 124, and the lightpasses through the pixels 124. Herein the term “pixels” also includescolor sub-pixels of a color display panel.

For a color display panel 102 including a plurality of interleaved colorsub-pixel arrays, each color sub-pixel array corresponding to a colorchannel of a plurality of color channels of an image to be displayed bythe display apparatus 850, the light source 808 may be a multi-colorlight source for providing the illuminating light of a color channel ofthe plurality of color channels. The MEMS 840 may be a part of anin-coupler for in-coupling the illuminating light 806 into the slab 804.The controller 880 may cause the multi-color light source to outputlight of different color channels one after another in acolor-sequential manner. The controller 880 may operate the MEMS 840such that different color channels are in-coupled at different angles,whereby a lateral position of the light spots 820 and the optical powerdensity peaks 828 depends on the color channel of the illuminating light806, and can be matched to a lateral position of a corresponding colorsub-pixel sub-array of the display panel 102. The controller 880 maytune the tunable microlens array 818 to adjust the focusing distancedepending on a current color channel of the illuminating light, tocompensate for Talbot distance wavelength dependence and/or chromaticaberration/chromatic focal shift of the tunable microlens array 818.

Referring now to FIG. 9 with further reference to FIG. 8 , a method 900for illuminating a display panel including a pixel array on a substrateincludes propagating (906) illuminating light in a slab of transparentmaterial e.g. the slab 804 (FIG. 8 ), by a series of internalreflections, such as total internal reflections (TIRs), from opposedsurfaces of the slab. Portions of the illuminating light are out-coupled(FIG. 9 ; 908) from the slab at one of the slab's outer surfaces usingan out-coupler. The out-coupler may include a grating disposed on orwithin the slab such as a surface-relief grating, a PVH grating, etc.The illuminating light portions are focused (910) at a distance from thesurface by a microlens array, e.g. the tunable microlens array 818 ofFIG. 8 . The focused light may be propagated further (912) through thesubstrate of the display panel to form an array of optical power densitypeaks due to Talbot effect. The microlens array may be tuned (914) toadjust the location of the focused light spots and/or the optical powerdensity peaks to match the plane of the display panel (sub)pixels.

For color display panels, the method 900 may include operating (902) amulti-color light source to provide the illuminating light in acolor-sequential manner, i.e. one color channel after another. Themicrolens tuning 914 may be performed in coordination with changinglight color of the illuminating light, by adjusting the focusingdistance so as to offset color-dependent focusing effects as explainedabove. Different color channels may be in-coupled (904) at differentangles, whereby a lateral position of light spots depends on the colorchannel of the illuminating light. A lateral position of the opticalpower density peaks may be matched (916) to a lateral position of acorresponding color sub-pixel sub-array of the color display panel. InFIG. 9 , optional steps are shown with dashed rounded boxes.

Referring to FIG. 10 , a virtual reality (VR) near-eye display 1000includes a frame 1001 supporting, for each eye: an illuminator 1030including any of the illuminators disclosed herein; a display panel 1010including an array of display pixels; and an ocular lens 1020 forconverting the image in linear domain generated by the display panel1010 into an image in angular domain for direct observation at an eyebox1012. A plurality of eyebox illuminators 1006, shown as black dots, maybe placed around the display panel 1010 on a surface that faces theeyebox 1012. An eye-tracking camera 1004 may be provided for each eyebox1012.

The purpose of the eye-tracking cameras 1004 is to determine positionand/or orientation of both eyes of the user. The eyebox illuminators1006 illuminate the eyes at the corresponding eyeboxes 1012, allowingthe eye-tracking cameras 1004 to obtain the images of the eyes, as wellas to provide reference reflections i.e. glints. The glints may functionas reference points in the captured eye image, facilitating the eyegazing direction determination by determining position of the eye pupilimages relative to the glints images. To avoid distracting the user withthe light of the eyebox illuminators 1006, the latter may be made toemit light invisible to the user. For example, infrared light may beused to illuminate the eyeboxes 1012.

Turning to FIG. 11 , an HMD 1100 is an example of an AR/VR wearabledisplay system which encloses the user's face, for a greater degree ofimmersion into the AR/VR environment. The HMD 1100 may generate theentirely virtual 3D imagery. The HMD 1100 may include a front body 1102and a band 1104 that can be secured around the user's head. The frontbody 1102 is configured for placement in front of eyes of a user in areliable and comfortable manner. A display system 1180 may be disposedin the front body 1102 for presenting AR/VR imagery to the user. Thedisplay system 1180 may include any of the display devices andilluminators disclosed herein. Sides 1106 of the front body 1102 may beopaque or transparent.

In some embodiments, the front body 1102 includes locators 1108 and aninertial measurement unit (IMU) 1110 for tracking acceleration of theHMD 1100, and position sensors 1112 for tracking position of the HMD1100. The IMU 1110 is an electronic device that generates dataindicating a position of the HMD 1100 based on measurement signalsreceived from one or more of position sensors 1112, which generate oneor more measurement signals in response to motion of the HMD 1100.Examples of position sensors 1112 include: one or more accelerometers,one or more gyroscopes, one or more magnetometers, another suitable typeof sensor that detects motion, a type of sensor used for errorcorrection of the IMU 1110, or some combination thereof. The positionsensors 1112 may be located external to the IMU 1110, internal to theIMU 1110, or some combination thereof.

The locators 1108 are traced by an external imaging device of a virtualreality system, such that the virtual reality system can track thelocation and orientation of the entire HMD 1100. Information generatedby the IMU 1110 and the position sensors 1112 may be compared with theposition and orientation obtained by tracking the locators 1108, forimproved tracking accuracy of position and orientation of the HMD 1100.Accurate position and orientation is important for presentingappropriate virtual scenery to the user as the latter moves and turns in3D space.

The HMD 1100 may further include a depth camera assembly (DCA) 1111,which captures data describing depth information of a local areasurrounding some or all of the HMD 1100. The depth information may becompared with the information from the IMU 1110, for better accuracy ofdetermination of position and orientation of the HMD 1100 in 3D space.

The HMD 1100 may further include an eye tracking system 1114 fordetermining orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes also allows the HMD 1100to determine the gaze direction of the user and to adjust the imagegenerated by the display system 1180 accordingly. The determined gazedirection and vergence angle may be used to adjust the display system1180 to reduce the vergence-accommodation conflict. The direction andvergence may also be used for displays' exit pupil steering as disclosedherein. Furthermore, the determined vergence and gaze angles may be usedfor interaction with the user, highlighting objects, bringing objects tothe foreground, creating additional objects or pointers, etc. An audiosystem may also be provided including e.g. a set of small speakers builtinto the front body 1102.

Embodiments of the present disclosure may include, or be implemented inconjunction with, an artificial reality system. An artificial realitysystem adjusts sensory information about outside world obtained throughthe senses such as visual information, audio, touch (somatosensation)information, acceleration, balance, etc., in some manner beforepresentation to a user. By way of non-limiting examples, artificialreality may include virtual reality (VR), augmented reality (AR), mixedreality (MR), hybrid reality, or some combination and/or derivativesthereof. Artificial reality content may include entirely generatedcontent or generated content combined with captured (e.g., real-world)content. The artificial reality content may include video, audio,somatic or haptic feedback, or some combination thereof. Any of thiscontent may be presented in a single channel or in multiple channels,such as in a stereo video that produces a three-dimensional effect tothe viewer. Furthermore, in some embodiments, artificial reality mayalso be associated with applications, products, accessories, services,or some combination thereof, that are used to, for example, createcontent in artificial reality and/or are otherwise used in (e.g.,perform activities in) artificial reality. The artificial reality systemthat provides the artificial reality content may be implemented onvarious platforms, including a wearable display such as an HMD connectedto a host computer system, a standalone HMD, a near-eye display having aform factor of eyeglasses, a mobile device or computing system, or anyother hardware platform capable of providing artificial reality contentto one or more viewers.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. An illuminator comprising: a slab of transparentmaterial, the slab comprising opposed first and second surfaces forpropagating illuminating light in the slab by a series of internalreflections from the first and second surfaces; an out-coupler supportedby the slab for out-coupling portions of the illuminating light from theslab at the first surface; and a tunable microlens array coupled to thefirst surface for forming an array of light spots from the out-coupledilluminating light portions at an adjustable distance from the firstsurface.
 2. The illuminator of claim 1, further comprising: amulti-color light source for providing the illuminating light of a colorchannel of a plurality of color channels; and an in-coupler forin-coupling the illuminating light into the slab, wherein the in-coupleris configured to in-couple different color channels of the plurality ofcolor channels at different angles, whereby a lateral position of lightspots of the array of light spots depends on the color channel of theilluminating light.
 3. The illuminator of claim 1, wherein the tunablemicrolens array comprises a liquid crystal layer with a variable liquidcrystal orientation.
 4. The illuminator of claim 1, wherein the tunablemicrolens array comprises a tunable liquid crystal microlens array. 5.The illuminator of claim 1, wherein the tunable microlens arraycomprises an array of switchable Pancharatnam-Berry phase microlenses.6. A display apparatus comprising: a display panel comprising a pixelarray on a substrate; and an illuminator coupled to the display panelfor illuminating the pixel array through the substrate, the illuminatorcomprising: a slab of transparent material, the slab comprising opposedfirst and second surfaces for propagating illuminating light in the slabby a series of internal reflections from the first and second surfaces;an out-coupler supported by the slab for out-coupling portions of theilluminating light from the slab at the first surface; and a tunablemicrolens array coupled to the first surface for forming an array oflight spots from the out-coupled illuminating light portions at adistance from the first surface.
 7. The display apparatus of claim 6,wherein in operation, the an array of light spots is formed on the pixelarray.
 8. The display apparatus of claim 6, wherein in operation, lightof the array of light spots propagates through the substrate andproduces an array of optical power density peaks at the pixel array dueto Talbot effect.
 9. The display apparatus of claim 6, wherein the pixelarray comprises a plurality of interleaved color sub-pixel arrays, eachcolor sub-pixel array corresponding to a color channel of a plurality ofcolor channels of an image to be displayed by the display apparatus, theilluminator further comprising: a multi-color light source for providingthe illuminating light of a color channel of the plurality of colorchannels; and an in-coupler for in-coupling the illuminating light intothe slab, wherein the in-coupler is configured to in-couple differentcolor channels of the plurality of color channels at different angles,whereby a lateral position of light spots of the array of light spotsdepends on the color channel of the illuminating light.
 10. The displayapparatus of claim 9, wherein in operation, light of the array of lightspots propagates through the substrate and produces an array of opticalpower density peaks at the pixel array due to Talbot effect, wherein alateral position of optical power density peaks of the array of opticalpower density peaks is matched to a lateral position of a correspondingcolor sub-pixel sub-array of the plurality of interleaved colorsub-pixel arrays.
 11. The display apparatus of claim 9, furthercomprising a controller operably coupled to the multi-color light sourceand the tunable microlens array and configured to: operate themulti-color light source to provide the illuminating light in acolor-sequential manner; and tune the tunable microlens array to adjustthe distance depending on a current color channel of the illuminatinglight.
 12. The display apparatus of claim 9, wherein the in-couplercomprises a tiltable reflector for varying an angle of incidence of theilluminating light onto the slab.
 13. The display apparatus of claim 6,wherein the tunable microlens array comprises a liquid crystal layerwith a variable liquid crystal orientation.
 14. The display apparatus ofclaim 6, wherein the tunable microlens array comprises a tunable liquidcrystal microlens array.
 15. The display apparatus of claim 6, whereinthe tunable microlens array comprises an array of switchablePancharatnam-Berry phase microlenses.
 16. A method for illuminating adisplay panel comprising a pixel array on a substrate, the methodcomprising: propagating illuminating light in a slab of transparentmaterial by a series of internal reflections from opposed first andsecond surfaces of the slab; out-coupling portions of the illuminatinglight from the slab at the first surface using an out-coupler; focusingthe out-coupled illuminating light portions at a distance from the firstsurface using a tunable microlens array; and tuning the tunablemicrolens array to form an array of light spots for illuminating thepixel array of the display panel.
 17. The method of claim 16, furthercomprising propagating light of the array of light spots through thesubstrate to produce an array of optical power density peaks at thepixel array due to Talbot effect.
 18. The method of claim 16, furthercomprising: operating a multi-color light source to provide theilluminating light in a color-sequential manner; and tuning the tunablemicrolens array to adjust the distance depending on a current color ofthe illuminating light.
 19. The method of claim 18, further comprisingin-coupling different color channels of the plurality of color channelsat different angles, whereby a lateral position of light spots of thearray of light spots depends on a color channel of the illuminatinglight.
 20. The method of claim 19, wherein the pixel array comprises aplurality of interleaved color sub-pixel arrays, the method furthercomprising: propagating light of the array of light spots through thesubstrate to produce an array of optical power density peaks at thepixel array due to Talbot effect; and matching a lateral position of theoptical power density peaks of the array of optical power density peaksto a lateral position of a corresponding color sub-pixel sub-array ofthe plurality of interleaved color sub-pixel arrays.